The present invention relates to methods of encoding position information, such as track addresses, for magnetic disk drives.
The position information about the recording portion of a recording medium is encoded and recorded in each recording portion for use in specifying the head position when data is read from or written into the recording portion. This will be described in more detail taking a magnetic disk for example.
In the magnetic disk, there are provided a plurality of concentric tracks, at a given track of which the magnetic head is positioned to write into or read data from the magnetic disk. Position or address information for positioning the magnetic head has been written in a portion of each track so that the magnetic head reads the address information for positioning. Sometimes, the address information is read during a seek period when the head moves over tracks. Consequently, the coding must be made so that reading errors are minimized. An example of such codes is Gray code described in Japanese Patent Application Kokai No. 55-55483 and Mini-Micro System, February 1983, pp. 219-226.
The Gray code used as an address information code will be described with reference to FIG. 11, wherein address information 10 is expressed in Gray code 11. A recording medium 1, such as a magnetic disk, contains clock information 2 which is expressed in magnetic inversion, and address information units or bits 3a and 3b which constitute a Gray code. The address information bits 3a and 3b represent "0" and "1" respectively. The clock information 2 and the coded address information bits 3a and 3b constitute address or position information 4. A magnetic head 5 moves along a locus A to read such magnetically recorded information.
The operation will be described with reference to FIG. 12. The magnetic head 5 reads out the address information 4 to provide an address signal 6 which is composed of a clock signal 7 (produced by reading the clock information 2 and address signal bits 8a and 8b (produced by reading the encoded address information bits 3a and (3b). The address signal bits 8a and 8b representing "0" and "1", respectively, constitute an address signal. It is apparent from the graph that the address information bit 8b read out within T/2 from the clock signal 7 is recognized as "1" while the address information bit 8a read out in a period between T/2 and T is recognized as "0", wherein T is the period of a clock signal. In this way, the address information 4 expressed in Gray code and recorded on the recording medium 1 is reproduced.
The case where the recording medium 1 has a defective spot 9 will be described with reference to FIG. 13. If no magnetic inversion is recorded on the defective spot 9, then the address signal 6 read by the magnetic head 5 moving along the locus A is such as shown in FIG. 14. This address signal 6 does not have one "1" signal bit 8b because of the defective spot 9. Consequently, it is impossible to determine whether the address information 10 is "00001" or "00101" and it is impossible to reproduce the encoded information. In this way, if one of address information bits in Gray code cannot be read because of a defective spot, the encoded address information cannot be reproduced.
To solve this problem, Japanese Patent Application Kokoku No. 58-501,644 has proposed that recording a Gray code be repeated three times as shown in FIG. 15. For address information 4, three pieces of indentical address information 4a, 4b, and 4c are written in sequence. The address signal 6 read by the magnetic head 5 moving along the locus A is shown in FIG. 16(a). The address information 4a does not have one "1" signal bit 8b because of the defective spot 9 and, therefore it cannot provide the encoded address information 4. However, the two other pieces of address information 4b and 4c read out are correct so that the encoded address information 4 is reproduced under majority rule as shown in FIG. 16(b).
In general, recording media do not have a plurality of defective spots in series, and the above method is able to prevent most of errors resulting from the defective medium. However, the number of bits for encoding address information by this method is large; namely, EQU l=3.times.log .sub.2 m
wherein l is the number of bits and m is the number of pieces of address information. For example, the number of bits for encoding 32 pieces of address information is EQU 3.times.log .sub.2 32=15 bits.
As has been described above, where Gray codes are used to encode address information, only one defective spot of a recording medium makes it impossible to reproduce the encoded address information, whereas if Gray codes are written three times, the number of bits used to encode address information becomes disadvantageously large.